The present invention generally relates to a single tool capable of acting as an inclinometer and rotational gauge, and more particularly to a tool and method for obtaining angular information such as the inclination and rotation of an object. The present invention also particularly relates to a borehole or well surveying tool for measuring deviations of the borehole or well from the vertical after or while drilling, and for determining the rotational orientation of a tool string in the borehole or well.
In drilling holes for exploration and extraction of oil or gas, it is highly desirable to determine the exact amount of deflection of the drilled hole from the vertical. Even after a borehole has been drilled, and when tool strings are inserted downhole it is often desirable to know both the angle of inclination of the borehole tool and the borehole itself. Indeed, while investigating a borehole, it is desirable to also determine the angular orientation of the tool within the borehole, as such a determination would permit a better circumferential resolution and hence a more complete analysis of the borehole. Moreover, even after the borehole has been cased, and production has begun, it is often desirable to determine the angle of inclination of the borehole and casing as the readings of many production tools may be affected by the inclination. For example, where multiphase fluids are flowing in a well, an inclination of as little as two degrees can cause a downflow or countercurrent flow in the pipe. Likewise, it may be important to determine the rotation of the tool in the well as some tools such as radioactive tracer tools require such knowledge.
Various inclinometers arranged especially for borehole use are known in the art. For example, U.S. Pat. No. 2,103,235, to J. Conrad discloses a device which includes a chamber which is supported by ball bearings. The chamber, which includes a pin extending therefrom always extends vertically due to gravitational forces. By taking photographs of the pin relative to a background which inclines with the direction of the borehole, a determination of the inclination may be had. U.S. Pat. No. 2,616,187, to E. Young discloses the use of a fluid with an indicating element therein to provide an inclination and compass reading downhole via the use of a camera. U.S. Pat. No. 3,400,464 to R. Karol uses the chamber having some fluid which causes the core rod members of transformers to move relative to the armature coils of the transformers. Because the surface of the fluid will always be perpendicular to the vertical, the relative movement of the rods is used to provide an electrical signal which can be converted via calibration into an inclination determination. In somewhat of a similar vein, U.S. Pat. No. 3,137,077 to Rosenthal discloses an inclinometer which has a chamber partially filled with liquid and with two electrical elements which are spaced near or on the circumference of the chamber. When the chamber is inclined from the vertical, the liquid maintains its surface which is perpendicular to the vertical. The difference in electrical resistance of the elements is then used with calibration to provide an inclination determination.
It will be recognized by those skilled in the art that the inclinometers of the art as described above have various drawbacks. With regard to the devices that use photography, it is evident that such an arrangement is not only undesirable, but is also subject to large errors. With regard to the disclosed devices which use liquid surfaces and electrical readings, it is stated in the patents that the disclosed devices cannot work when there are torsional (rotational) movements of the tool. Thus, in order for proper measurements to be made, the chambers in the tools must be kept rotationally unvarying, thereby introducing other limitations and difficulties into the situation. Also, it will be appreciated that where the chambers are only partially filled with liquid, any bumping of the tool will cause a sloshing of the liquid, thereby either requiring a large settling time, or causing an inaccurate reading.
Another inclinometer is disclosed in U.S. Pat. No. 3,478,437 to W. Cothran. While the inclinometer was not disclosed with reference to use in a borehole, it does disclose a device which permits rotation. Thus, several conductor rings are placed circumferentially around a chamber which is partially filled with a conducting liquid. When the chamber is tilted, more conductors are brought into contact with the liquid, in effect acting as a short between some of the conductor rings. With each ring being connected to a resistor, the change in resistance permits a determination of the inclination of the chamber. However, because each ring must be separated from the others, the provided device only permits the inclination to be determined in finite steps, and such a poor resolution is often not adequate. The device, if it could be placed in a borehole setting, would also suffer from the sloshing problem common to the other liquid filled devices.
Finally, it should be noted that the patent to Rosenthal, U.S. Pat. No. 3,137,077 includes an embodiment as shown in FIG. 2 thereof which permits rotation of the inclinometer. The embodiment utilizes six elements rather than the two of the above-described embodiment and thereby permits two of the elements to be approximately properly placed to make an inclination determination. However, additional complication and error is introduced by such a six element design as the elements used in making the inclination determination may not be optimally located and interpolation would be required. Moreover, no measure of rotation is accomplished by such an arrangement.